In recent years, an RFID (Radio Frequency Identification) system has been widely used for various purposes. The RFID system includes a wireless tag and a reader, and performs various functions via wireless communications between the tag and the reader.
The wireless tag for use in the RFID system is categorized into a passive tag that contains no battery and an active tag that contains a battery. The passive tag is used as a wireless tag for wireless communications between itself and a reader that is close to it (e.g., for use as a pre-paid card). On the other hand, the active tag is used as a wireless tag for wireless communications between itself and a reader that is not close to it (e.g., for use as a tag carried by a user in a presence management system or as a tag attached to a commercial product in an inventory management system). The presence management system is disclosed in, for example, Patent Literature 1.
It is necessary that the wireless tag include an antenna for wireless communications between itself and a reader. As the antenna included in the wireless tag, an antenna including a linear antenna element is often used, such as a loop antenna, a dipole antenna or a monopole antenna.
However, when such an antenna is mounted in or on the wireless tag, it may be necessary that the antenna element be positioned so as to run along an edge of the wireless tag. In particular, in a case where it is desired that the antenna operate in a low frequency band (low resonant frequency), the total length of the antenna element is necessarily long. Therefore, in such a case, the antenna element is more likely to be positioned so as to run along an edge of the wireless tag. However, edges of the wireless tag often make contact with an external conductor (e.g., user's finger). In this case, if the antenna element is positioned so as to run along an edge of the wireless tag, the external conductor may interfere with an electric field around the antenna element and thereby antenna characteristics may deteriorate.
As an antenna less prone to this problem, there has been known a notch antenna constituted by a conductor plate having a notch. As a notch antenna that can be mounted in or on a wireless tag, for example, there has been known an antenna disclosed in Patent Literature 2. The notch antenna is a planar antenna. Also because of this, the notch antenna is suitable for use in a card-shaped active tag.